The control of subsea well head equipment from a platform includes the use of multiple hydraulic control lines or subsea sequential valves to minimize the number of control lines extending to the subsea well head. Having a single control line leading to the subsea well head is advantageous provided that its operation of each of a number of pieces of equipment is assured. Difficulty has been encountered in the past with sequential valves having a dead space or inoperative region so that pressures which should switch the valve to a certain control function may not be quite sufficient, because of friction losses, and therefore, the desired function may not be turned on.
A summary of the use of such sequential valves is set forth in an article entitled "Sequential Hydraulic Valve Controls All Production Functions Through One Line" in the January 1977 issue of Ocean Industry.
The Benton F. Baugh U.S. Pat. No. 3,952,763 shows one example of the prior art sequential control valve. The Vivian H. Payne U.S. Pat. No. 3,683,966 shows an example of a multiple position valve using shear valves to cover or uncover ports extending through the valve body and being shifted with the piston in which they are mounted responsive to pilot pressure on one end of the piston and operational pressure on the other end of the piston.